When a regexp is compiled, its engine field is then set to point at the appropriate structure, so that when it needs to be used Perl can find the right routines to do so.

In order to install a new regexp handler, $^H{regcomp} is set to an integer which (when casted appropriately) resolves to one of these structures. When compiling, the comp method is executed, and the resulting regexp structure's engine field is expected to point back at the same structure.

The pTHX_ symbol in the definition is a macro used by Perl under threading to provide an extra argument to the routine holding a pointer back to the interpreter that is executing the regexp. So under threading all routines get an extra argument.

Callbacks

comp

REGEXP* comp(pTHX_ const SV * const pattern, const U32 flags);

Compile the pattern stored in pattern using the given flags and return a pointer to a prepared REGEXP structure that can perform the match. See "The REGEXP structure" below for an explanation of the individual fields in the REGEXP struct.

The pattern parameter is the scalar that was used as the pattern. Previous versions of Perl would pass two char* indicating the start and end of the stringified pattern; the following snippet can be used to get the old parameters:

Since any scalar can be passed as a pattern, it's possible to implement an engine that does something with an array ("ook" =~ [ qw/ eek hlagh / ]) or with the non-stringified form of a compiled regular expression ("ook" =~ qr/eek/). Perl's own engine will always stringify everything using the snippet above, but that doesn't mean other engines have to.

The flags parameter is a bitfield which indicates which of the msixpn flags the regex was compiled with. It also contains additional info, such as if use locale is in effect.

The eogc flags are stripped out before being passed to the comp routine. The regex engine does not need to know if any of these are set, as those flags should only affect what Perl does with the pattern and its match variables, not how it gets compiled and executed.

By the time the comp callback is called, some of these flags have already had effect (noted below where applicable). However most of their effect occurs after the comp callback has run, in routines that read the rx->extflags field which it populates.

In general the flags should be preserved in rx->extflags after compilation, although the regex engine might want to add or delete some of them to invoke or disable some special behavior in Perl. The flags along with any special behavior they cause are documented below:

The pattern modifiers:

/m - RXf_PMf_MULTILINE

If this is in rx->extflags it will be passed to Perl_fbm_instr by pp_split which will treat the subject string as a multi-line string.

/s - RXf_PMf_SINGLELINE

/i - RXf_PMf_FOLD

/x - RXf_PMf_EXTENDED

If present on a regex, "#" comments will be handled differently by the tokenizer in some cases.

TODO: Document those cases.

/p - RXf_PMf_KEEPCOPY

TODO: Document this

Character set

The character set rules are determined by an enum that is contained in this field. This is still experimental and subject to change, but the current interface returns the rules by use of the in-line function get_regex_charset(const U32 flags). The only currently documented value returned from it is REGEX_LOCALE_CHARSET, which is set if use locale is in effect. If present in rx->extflags, split will use the locale dependent definition of whitespace when RXf_SKIPWHITE or RXf_WHITE is in effect. ASCII whitespace is defined as per isSPACE, and by the internal macros is_utf8_space under UTF-8, and isSPACE_LC under use locale.

Additional flags:

RXf_SPLIT

This flag was removed in perl 5.18.0. split ' ' is now special-cased solely in the parser. RXf_SPLIT is still #defined, so you can test for it. This is how it used to work:

If split is invoked as split ' ' or with no arguments (which really means split(' ', $_), see split), Perl will set this flag. The regex engine can then check for it and set the SKIPWHITE and WHITE extflags. To do this, the Perl engine does:

These flags can be set during compilation to enable optimizations in the split operator.

RXf_SKIPWHITE

This flag was removed in perl 5.18.0. It is still #defined, so you can set it, but doing so will have no effect. This is how it used to work:

If the flag is present in rx->extflagssplit will delete whitespace from the start of the subject string before it's operated on. What is considered whitespace depends on if the subject is a UTF-8 string and if the RXf_PMf_LOCALE flag is set.

If RXf_WHITE is set in addition to this flag, split will behave like split " " under the Perl engine.

RXf_START_ONLY

Tells the split operator to split the target string on newlines (\n) without invoking the regex engine.

Perl's engine sets this if the pattern is /^/ (plen == 1 && *exp == '^'), even under /^/s; see split. Of course a different regex engine might want to use the same optimizations with a different syntax.

RXf_WHITE

Tells the split operator to split the target string on whitespace without invoking the regex engine. The definition of whitespace varies depending on if the target string is a UTF-8 string and on if RXf_PMf_LOCALE is set.

Perl's engine sets this flag if the pattern is \s+.

RXf_NULL

Tells the split operator to split the target string on characters. The definition of character varies depending on if the target string is a UTF-8 string.

Perl's engine sets this flag on empty patterns, this optimization makes split // much faster than it would otherwise be. It's even faster than unpack.

RXf_NO_INPLACE_SUBST

Added in perl 5.18.0, this flag indicates that a regular expression might perform an operation that would interfere with inplace substitution. For instance it might contain lookbehind, or assign to non-magical variables (such as $REGMARK and $REGERROR) during matching. s/// will skip certain optimisations when this is set.

intuit

Find the start position where a regex match should be attempted, or possibly if the regex engine should not be run because the pattern can't match. This is called, as appropriate, by the core, depending on the values of the extflags member of the regexp structure.

Arguments:

rx: the regex to match against
sv: the SV being matched: only used for utf8 flag; the string
itself is accessed via the pointers below. Note that on
something like an overloaded SV, SvPOK(sv) may be false
and the string pointers may point to something unrelated to
the SV itself.
strbeg: real beginning of string
strpos: the point in the string at which to begin matching
strend: pointer to the byte following the last char of the string
flags currently unused; set to 0
data: currently unused; set to NULL

checkstr

SV* checkstr(pTHX_ REGEXP * const rx);

Return a SV containing a string that must appear in the pattern. Used by split for optimising matches.

free

void free(pTHX_ REGEXP * const rx);

Called by Perl when it is freeing a regexp pattern so that the engine can release any resources pointed to by the pprivate member of the regexp structure. This is only responsible for freeing private data; Perl will handle releasing anything else contained in the regexp structure.

Numbered capture callbacks

Called to get/set the value of $`, $', $& and their named equivalents, ${^PREMATCH}, ${^POSTMATCH} and ${^MATCH}, as well as the numbered capture groups ($1, $2, ...).

The paren parameter will be 1 for $1, 2 for $2 and so forth, and have these symbolic values for the special variables:

Note that in Perl 5.17.3 and earlier, the last three constants were also used for the caret variants of the variables.

The names have been chosen by analogy with Tie::Scalar methods names with an additional LENGTH callback for efficiency. However named capture variables are currently not tied internally but implemented via magic.

numbered_buff_FETCH

Fetch a specified numbered capture. sv should be set to the scalar to return, the scalar is passed as an argument rather than being returned from the function because when it's called Perl already has a scalar to store the value, creating another one would be redundant. The scalar can be set with sv_setsv, sv_setpvn and friends, see perlapi.

This callback is where Perl untaints its own capture variables under taint mode (see perlsec). See the Perl_reg_numbered_buff_fetch function in regcomp.c for how to untaint capture variables if that's something you'd like your engine to do as well.

Actually Perl will not always croak in a statement that looks like it would modify a numbered capture variable. This is because the STORE callback will not be called if Perl can determine that it doesn't have to modify the value. This is exactly how tied variables behave in the same situation:

Because $sv is undef when the y/// operator is applied to it, the transliteration won't actually execute and the program won't die. This is different to how 5.8 and earlier versions behaved since the capture variables were READONLY variables then; now they'll just die when assigned to in the default engine.

numbered_buff_LENGTH

Get the length of a capture variable. There's a special callback for this so that Perl doesn't have to do a FETCH and run length on the result, since the length is (in Perl's case) known from an offset stored in rx->offs, this is much more efficient:

This is a little bit more complex in the case of UTF-8, see what Perl_reg_numbered_buff_length does with is_utf8_string_loclen.

Named capture callbacks

Called to get/set the value of %+ and %-, as well as by some utility functions in re.

There are two callbacks, named_buff is called in all the cases the FETCH, STORE, DELETE, CLEAR, EXISTS and SCALAR Tie::Hash callbacks would be on changes to %+ and %- and named_buff_iter in the same cases as FIRSTKEY and NEXTKEY.

The flags parameter can be used to determine which of these operations the callbacks should respond to. The following flags are currently defined:

Which Tie::Hash operation is being performed from the Perl level on %+ or %+, if any:

If this is being called as re::regname, re::regnames or re::regnames_count, if any. The first two will be combined with RXapif_ONE or RXapif_ALL.

RXapif_REGNAME
RXapif_REGNAMES
RXapif_REGNAMES_COUNT

Internally %+ and %- are implemented with a real tied interface via Tie::Hash::NamedCapture. The methods in that package will call back into these functions. However the usage of Tie::Hash::NamedCapture for this purpose might change in future releases. For instance this might be implemented by magic instead (would need an extension to mgvtbl).

named_buff

named_buff_iter

qr_package

SV* qr_package(pTHX_ REGEXP * const rx);

The package the qr// magic object is blessed into (as seen by ref qr//). It is recommended that engines change this to their package name for identification regardless of if they implement methods on the object.

The package this method returns should also have the internal Regexp package in its @ISA. qr//->isa("Regexp") should always be true regardless of what engine is being used.

dupe

void* dupe(pTHX_ REGEXP * const rx, CLONE_PARAMS *param);

On threaded builds a regexp may need to be duplicated so that the pattern can be used by multiple threads. This routine is expected to handle the duplication of any private data pointed to by the pprivate member of the regexp structure. It will be called with the preconstructed new regexp structure as an argument, the pprivate member will point at the old private structure, and it is this routine's responsibility to construct a copy and return a pointer to it (which Perl will then use to overwrite the field as passed to this routine.)

This allows the engine to dupe its private data but also if necessary modify the final structure if it really must.

On unthreaded builds this field doesn't exist.

op_comp

This is private to the Perl core and subject to change. Should be left null.

The REGEXP structure

The REGEXP struct is defined in regexp.h. All regex engines must be able to correctly build such a structure in their "comp" routine.

The REGEXP structure contains all the data that Perl needs to be aware of to properly work with the regular expression. It includes data about optimisations that Perl can use to determine if the regex engine should really be used, and various other control info that is needed to properly execute patterns in various contexts, such as if the pattern anchored in some way, or what flags were used during the compile, or if the program contains special constructs that Perl needs to be aware of.

In addition it contains two fields that are intended for the private use of the regex engine that compiled the pattern. These are the intflags and pprivate members. pprivate is a void pointer to an arbitrary structure, whose use and management is the responsibility of the compiling engine. Perl will never modify either of these values.

engine

This field points at a regexp_engine structure which contains pointers to the subroutines that are to be used for performing a match. It is the compiling routine's responsibility to populate this field before returning the regexp object.

Internally this is set to NULL unless a custom engine is specified in $^H{regcomp}, Perl's own set of callbacks can be accessed in the struct pointed to by RE_ENGINE_PTR.

mother_re

extflags

This will be used by Perl to see what flags the regexp was compiled with, this will normally be set to the value of the flags parameter by the comp callback. See the comp documentation for valid flags.

minlenminlenret

The minimum string length (in characters) required for the pattern to match. This is used to prune the search space by not bothering to match any closer to the end of a string than would allow a match. For instance there is no point in even starting the regex engine if the minlen is 10 but the string is only 5 characters long. There is no way that the pattern can match.

minlenret is the minimum length (in characters) of the string that would be found in $& after a match.

The difference between minlen and minlenret can be seen in the following pattern:

/ns(?=\d)/

where the minlen would be 3 but minlenret would only be 2 as the \d is required to match but is not actually included in the matched content. This distinction is particularly important as the substitution logic uses the minlenret to tell if it can do in-place substitutions (these can result in considerable speed-up).

gofs

Left offset from pos() to start match at.

substrs

Substring data about strings that must appear in the final match. This is currently only used internally by Perl's engine, but might be used in the future for all engines for optimisations.

nparens, lastparen, and lastcloseparen

These fields are used to keep track of how many paren groups could be matched in the pattern, which was the last open paren to be entered, and which was the last close paren to be entered.

intflags

The engine's private copy of the flags the pattern was compiled with. Usually this is the same as extflags unless the engine chose to modify one of them.

pprivate

A void* pointing to an engine-defined data structure. The Perl engine uses the regexp_internal structure (see "Base Structures" in perlreguts) but a custom engine should use something else.

offs

A regexp_paren_pair structure which defines offsets into the string being matched which correspond to the $& and $1, $2 etc. captures, the regexp_paren_pair struct is defined as follows:

If ->offs[num].start or ->offs[num].end is -1 then that capture group did not match. ->offs[0].start/end represents $& (or ${^MATCH} under /p) and ->offs[paren].end matches $$paren where $paren = 1>.

precompprelen

Used for optimisations. precomp holds a copy of the pattern that was compiled and prelen its length. When a new pattern is to be compiled (such as inside a loop) the internal regcomp operator checks if the last compiled REGEXP's precomp and prelen are equivalent to the new one, and if so uses the old pattern instead of compiling a new one.

paren_names

This is a hash used internally to track named capture groups and their offsets. The keys are the names of the buffers the values are dualvars, with the IV slot holding the number of buffers with the given name and the pv being an embedded array of I32. The values may also be contained independently in the data array in cases where named backreferences are used.

substrs

Holds information on the longest string that must occur at a fixed offset from the start of the pattern, and the longest string that must occur at a floating offset from the start of the pattern. Used to do Fast-Boyer-Moore searches on the string to find out if its worth using the regex engine at all, and if so where in the string to search.

subbegsublensaved_copysuboffsetsubcoffset

Used during the execution phase for managing search and replace patterns, and for providing the text for $&, $1 etc. subbeg points to a buffer (either the original string, or a copy in the case of RX_MATCH_COPIED(rx)), and sublen is the length of the buffer. The RX_OFFS start and end indices index into this buffer.

In the presence of the REXEC_COPY_STR flag, but with the addition of the REXEC_COPY_SKIP_PRE or REXEC_COPY_SKIP_POST flags, an engine can choose not to copy the full buffer (although it must still do so in the presence of RXf_PMf_KEEPCOPY or the relevant bits being set in PL_sawampersand). In this case, it may set suboffset to indicate the number of bytes from the logical start of the buffer to the physical start (i.e. subbeg). It should also set subcoffset, the number of characters in the offset. The latter is needed to support @- and @+ which work in characters, not bytes.

wrappedwraplen

Stores the string qr// stringifies to. The Perl engine for example stores (?^:eek) in the case of qr/eek/.

When using a custom engine that doesn't support the (?:) construct for inline modifiers, it's probably best to have qr// stringify to the supplied pattern, note that this will create undesired patterns in cases such as:

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